This invention relates to a control system for controlling the power output of a modulated laser diode.
It relates particularly to a control system for controlling the power output of a laser diode of the kind used in printers where the modulation of the light output may be 100% modulation between full on and full off and where there is a need to keep stable the value of the current needed to turn the laser diode on to the desired power level after relatively long periods of time when the laser diode is off and during which long periods of time no updating information is generated for the power control system.
In a printer environment a laser diode is usually modulated between a full on and a full off condition and may be required to produce a single pixel of light followed by a relatively long period in the full off condition before being modulated to another on condition.
A laser diode is a current control device that has poor long time stability characteristics. The stability characteristics vary with temperature and with age of the laser diode. For these reasons, the laser diode always has to be actively controlled.
The modulation of a laser diode in a printer is determined by the printing content. It is necessary to take and to sample the light every time the light is turned on. The sample is held electronically so that the changes in the modulation pattern do not, affect the output of the error signal of the control.
The control must respond within a reasonable time both to an increase and to a decrease in the signal to be evaluated in order to provide effective control of the power output of the laser diode.